Methods and systems for the reduction of molecules using diamond as a photoreduction catalyst

ABSTRACT

Methods for the photoreduction of molecules are provided. The methods use diamond having a negative electron affinity as a photocatalyst, taking advantage of its ability to act as a solid-state electron emitter that is capable of inducing reductions without the need for reactants to adsorb onto its surface. The methods comprise illuminating a fluid sample comprising the molecules to be reduced and hydrogen surface-terminated diamond having a negative electron affinity with light comprising a wavelength that induces the emission of electrons from the diamond directly into the fluid sample. The emitted electrons induce the reduction of the molecules to form a reduction product.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/362,819 that was filed Jan. 31, 2012, the entire contents ofwhich are hereby incorporated by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under 0911543 and0520527 awarded by the National Science Foundation. The government hascertain rights in the invention.

BACKGROUND

The reduction of small molecules, such as nitrogen and carbon dioxide,is extraordinarily difficult because the one-electron reductionprocesses often involve high-energy intermediates. For example, in thefixation of nitrogen (conversion of N₂ to NH₃), the reaction N₂+e⁻→N₂ ⁻involves such high energy that the gas-phase anion N₂ ⁻ exists only as afleeting transient. The standard reduction potential for the analogoussolution-phase reaction N₂+e⁻→N₂ ⁻ _((aq)) has been estimated at −4.2 Vvs. the normal hydrogen electrode (NHE). While some evidence exists forformation of N₂ ⁻ species at surfaces of ionic oxides such as MgO,nitrogen reduction is usually accomplished by coupling with the transferof one more protons. Yet, even these have high energy; the reactionN₂+H⁺+e⁻→N₂H, has a calculated reduction potential E° of −3.2 V vs. thenormal hydrogen electrode (NHE).

The photocatalytic reduction of nitrogen was first discovered bySchrauzer and Guth (G. N. Schrauzer, T. D. Guth, Journal of the AmericanChemical Society 99, 7189 (1977)), who showed that N₂ could be reducedto NH₃ on the surface of TiO₂ powder when illuminated with light from amercury arc lamp. Although since then various modified TiO₂ catalystshave been developed, the overall efficiency of the reaction remainspoor. The poor efficiency arises because the proton-coupled reactionshave relatively complicated pathways, and because the highly stable N₂molecule has only a very low binding affinity for surfaces.

SUMMARY

Methods for the photoreduction of molecules are provided. The methodsuse diamond having a negative electron affinity as a photocatalyst,taking advantage of its ability to act as a solid-state electron emitterthat is capable of inducing reductions without the need for reactants toadsorb onto its surface.

In one basic embodiment, the method comprises illuminating a fluidsample comprising the molecules to be reduced and hydrogensurface-terminated diamond having a negative electron affinity withlight comprising a wavelength that induces the emission of electronsfrom the diamond directly into the fluid sample, wherein the emittedelectrons induce the reduction of the molecules to form a reductionproduct. Once produced, the reduction product can be separated from thefluid sample and collected.

The methods can be used to reduce a variety of molecules, includingsmall molecules, such as N₂, CO₂, CO, NO_(x) and aromatic molecules thatinclude one or more benzene rings.

Also provided are reaction systems for carrying out the methods. In oneembodiment the reaction system comprises a reduction cell comprising afluid sample comprising the molecules to be reduced and hydrogensurface-terminated diamond having a negative electron affinity; and alight source configured to illuminate at least a portion of thereduction cell with light comprising a wavelength capable of inducingthe emission of electrons from the diamond into the fluid sample toinduce the reduction of the molecules to form a reduction product. Thesystem may further include a reduction product collection cellconfigured to collect the reduction product emitted from the reductioncell.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be described.

FIG. 1 is an electronic energy-level diagram showing the valence band(vb) and conduction band (cb) of H-terminated diamond and severalrelevant electrochemical reduction potentials, including both theabsolute energy scale (left) and the electrochemical energy scale(right) relative to the normal hydrogen electrode (NHE).

FIG. 2 shows graphs of the total ammonia yield from the photoreductionof N₂: a) boron-doped electrochemical grade diamond in an H-cell and asingle-cell measured in N₂-saturated water; (b) natural diamond powder,c) Ru/TiO₂ and d) electronic-grade diamond.

FIG. 3 shows (a) the ammonia yield from boron-doped diamond in an H-cellafter illumination for 1 hour, using absorptive filters to limit therange of incident radiation; (b) the time-resolved electron emissionfrom boron-doped diamond into N₂-saturated deionized water whenilluminated at 210 nm at room temperature; and c) the wavelengthdependence of the electron emission, normalized by wavelength to yieldan effective wavelength-dependent photoemission efficiency.

FIG. 4 shows (a) a comparison of NH₃ yield from H-terminated andO-terminated diamond samples, measured after illumination for 2 hours;(b) a comparison of XPS spectra of H-terminated and O-terminatedboron-doped diamond illuminated for 2 hours and for 3 days; and (c) theposition of the valence bands (vb) and the conduction bands (cb) ofboron-doped diamond as determined by ultraviolet photoemissionspectroscopy measurements, showing a transition from negative electronaffinity to positive electron affinity.

DETAILED DESCRIPTION

Methods and systems for the photocatalytic reduction of molecules areprovided. The methods utilize the ability of hydrogen-terminated diamondsurfaces to act as solid-state electron emitters able to inducereduction reactions.

Diamond is a wide-bandgap semiconductor with a bandgap of 5.5 eV. Whenthe surface of diamond is terminated with hydrogen atoms, theconduction-band energy lies ˜0.8-1.3 eV above the “vacuum level”, whichrepresents the energy of a free electron with zero kinetic energy. Thisproperty is termed ‘negative electron affinity’ (NEA). One consequenceof NEA is that when H-terminated diamond surfaces are illuminated withlight with photon energies hν greater than the bandgap, electrons thatare excited to the conduction band in the bulk can be directly emittedinto vacuum with no barrier. In effect, diamond becomes a directsolid-state emitter of electrons with energy equal to the conductionband of diamond. When placed on an electrochemical energy scale, asdepicted in FIG. 1, (using 4.44 eV as the absolute energy of the NHE)the emitted electrons have a reduction potential of approximately −5.5Volts vs. NHE. The high energy combined with diamond's NEA makes itpossible for illuminated diamond surfaces to emit electrons directlyinto adjacent reactants, thereby eliminating the need for reactantmolecules to adsorb onto the surface in order to perform photochemicalreduction reactions. In effect, the NEA property allows one to bringelectrons to the reactants, rather than having to bring the reactants tothe source of the electrons. Moreover, the high energy level of emittedelectrons makes it feasible to initiate very high-energy reductionprocesses.

One embodiment of the present methods comprises illuminating a fluidsample comprising the molecules to be reduced and the hydrogensurface-terminated diamond having a negative electron affinity withlight comprising a wavelength that induces the emission of electronsfrom the diamond into the fluid sample. The emitted electrons induce thereduction of the molecules to form a reduction product which can beseparated from the fluid sample and collected.

The present methods can be used to reduce a variety of molecules, suchas those capable of reduction via a one-electron reduction process orthose that undergo proton-coupled electron transfer processes. Themethods are particularly well-suited for the reduction of smallmolecules whose one-electron reduction processes involve high-energyintermediates. Reduction reactions that can be carried out using thepresent methods include, but are not limited to, the reduction of N₂ toNH₃ or hydrazine (N₂H₄); the reduction of CO₂ to CO, or organicmolecules such as methane (CH₄), formaldehyde (H₂CO) or methanol(CH₃OH), and the reduction of nitrogen oxides (NO_(x), i.e., NO and NO₂)to N₂. Other molecules that can be reduced using the present methodsinclude benzene ring-containing organic molecules of the type that arereducible via Birch reduction. Examples of such molecules includesubstituted and unsubstituted benzene and naphthalene.

The reductions may be single-step reductions or multiple (e.g., two ormore) step reductions. For example, the present methods can be used toreduce CO₂ to CO, which can be further reduced to other reductionproducts, such as CH₄, H₂CO and/or CH₃OH. Alternatively, theintermediate reduction products in a multiple-step reduction scheme canthemselves be used as the starting product in a single-step reduction.For example, rather than starting with CO₂, CO can be used as a startingproduct in a single-step reduction scheme for the production of CH₄,H₂CO and/or CH₃OH.

For the purposes of this disclosure, the term diamond refers to carbonmaterials, wherein the carbon atoms are primarily sp³ hybridized, andincludes species of diamond having varying degrees of crystallinity.Suitable carbon materials having sp³ hybridization ≧50% includemicrocrystalline diamond, nanocrystalline diamond, ultrananocrystallinediamond and diamond-like materials, such as tetrahedral amorphous carbon(ta-C).

The diamond may be doped or undoped. Undoped diamond has a strongabsorption in the UV region of the electromagnetic spectrum. Inembodiments where the diamond is doped, the dopants can be used toenhance the absorption of light in the visible and/or near UV regions ofthe spectrum, thereby providing a higher photocatalytic activity perunit area. The dopants can be p-type dopants, such as boron (B) atoms,or n-type dopants, such as phosphorus (P) atoms or nitrogen (N) atoms.

Nitrogen-doped diamond may be particularly well-suited for use in thepresent methods because previous studies have shown that electrons canbe field-emitted from nitrogen-doped diamond at very low energies, andthat these electrons are emitted from the conduction band rather thanthat valence band. (K. Okano, S. Koizumi, S. R. P. Silva, and G. A. J.Amaratunga, Nature (London) 381, 140 (1996); H. Yamaguchi, T. Masuzawa,S. Nozue, Y. Kudo, I. Saito, J. Koe, M. Kudo, T. Yamada, Y. Takakuwa,and K. Okano, Phys. Rev. B 80, 165321 (2009).) Consequently, electronsmay be emitted from nitrogen-doped diamond using longer wavelengths,including visible light. Recently Nemanich, et al. have reported thatelectrons can be emitted from nitrogen-doped diamond into vacuum usingvisible-light wavelengths of 340-550 nm. (T. Y. Sun, F. A. M. Koeck, C.Y. Zhu, and R. J. Nemanich, Applied Physics Letters, Volume: 99 Issue:20 Article Number: 202101 DOI: 10.1063/1.3658638 Published: Nov. 14,2011.) Consequently, the use of diamond doped with nitrogen or othern-type dopants such as phosphorus may be useful in providingphotocatalytic activity using visible light instead of ultravioletlight.

The diamond can be provided in the form of a powder that forms asuspension in the fluid sample. Suitable diamond powders include thosewith nominal average particle sizes in the range from about 100 nm toabout 300 nm. Although, powders with particles sizes outside of thisrange can also be used. Alternatively, the diamond can be coated orloaded on a support substrate to provide a diamond electrode that can beimmersed in the fluid sample. In yet another design, a diamond electrodeis itself formed entirely from diamond. The diamond can be ahigh-quality, electronics-grade diamond. However, such high qualitydiamond is not required. Lower grade diamond, including relativelyinexpensive diamond grit can also be used.

The fluid of the fluid sample in which the reductions are carried outwill typically be a liquid or supercritical fluid. In some embodiments,the fluid is the liquid or supercritical fluid form of a reductionreactant. For example, in the reduction of CO₂, the fluid of the fluidsample can be liquid CO₂ or supercritical CO₂. In other embodiments, thefluid of the fluid sample comprises a solvent for the molecules to bereduced. Examples of liquid media that can provide a solvent for themolecules to be reduced and/or other reactants include water, aqueoussolutions or organic solvent-based solutions. Suitable organic solventsinclude those in which the molecules to be reduced have significantsolubility. Examples of suitable organic solvents include propylenecarbonate (PC), dimethyl formamide (DMF) and methanol, in which CO₂ hassubstantial solubility. In addition, it is advantageous if the solventis able to stabilize the solvated electrons emitted from the diamond. Anexample of an organic solvent in which solvated electrons are verystable is hexamethylphosphoric triamide (HMPA). The molecules to bereduced can be introduced into the fluid along with other reactants,such as H₂. Gaseous reactants can be introduced into the fluid byflowing or bubbling them through the fluid sample. In some embodimentsthe fluid sample is saturated with the reactant in order to maximize theproduct yield.

The reduction reactions can be carried out in a system comprising areduction cell; a light source configured to illuminate at least aportion of the reduction cell with light; and a reduction productcollection cell configured to collect reduction product emitted from thereduction cell. In this system, the reduction cell comprises a fluidsample comprising the reactant molecules to be reduced and the hydrogensurface-terminated diamond having a negative electron affinity. Thelight source is selected to emit radiation having a wavelength capableof inducing the emission of electrons from the diamond into the fluidsample where they induce the reduction of the reactant molecules to forma reduction product. Generally, the light source will be an ultravioletlight source that emits light with wavelengths of about 230 nm(corresponding to the 5.5 eV bandgap energy of diamond) or lower. Ifvisible light is used wavelengths of, for example, ≦550 nm may be used(e.g., wavelengths in the range from about 440 to about 550 nm).However, if doped diamond is utilized, it may be advantageous to use alight source that emits across one or more of the UV, near-UV andvisible regions of the electromagnetic spectrum. Broadband lightsources, such as Xe arc lamps and HgXe arc lamps are suitable.Optionally, filters can be used to filter out unwanted radiation, suchas infrared radiation.

In embodiments where a diamond electrode is utilized, a counterelectrode can also be immersed in the liquid sample. In suchembodiments, a voltage source in electrical communication with theelectrodes can be used to apply a voltage between the diamond electrodeand the counter electrode. This small voltage can help ‘push’ theelectrons into the liquid sample and away from the diamond electrodeafter they are emitted. The applied voltage is desirably small (e.g., ≦2V) such that it does not interfere with the photocatalytic nature of thereduction process.

In some embodiments, the reduction system includes a single reductioncell geometry, while in other embodiments an H-cell geometry is used. Inthe single reduction cell geometry, the diamond in the reduction cellhas no external electrical connection and, therefore, must induce bothoxidation and reduction reactions in order to achieve charge neutrality.In the H-cell geometry a diamond electrode is immersed in the fluidsample in a first reduction cell and a counter electrode (e.g., aplatinum electrode) that is electrically connected to the diamondelectrode is immersed in an oxidation medium is a second cell (theoxidation cell). In this geometry the fluid sample and the oxidationmedium are in electrical contact but do not mix and the oxidation andreduction reactions are separated.

Once the reduction product is formed, it can be separated from the fluidsample and captured. For example, gas phase reduction product moleculeswill be generated as a gaseous effluent that can be collected in acollection cell after it escapes the fluid sample.

EXAMPLES Example 1

This example demonstrates the use of hydrogen-terminated diamond as aphotoreduction catalyst for the reduction of N₂ to NH₃.

Materials and Methods:

Samples:

Three different types of diamond were investigated. Electrochemistrygrade boron-doped diamond (Product 145-500-0030) and electronic-gradepolycrystalline diamond plates (Product 145-500-0005) were purchasedfrom Element Six, Inc. Natural diamond powder was purchased fromMicrodiamant AG, Lengwil Switzerland, 125 nm nominal average diameter,product NAT 0-0.25. In each case the diamond samples werehydrogen-terminated before use as described below.

Hydrogen-Termination:

Solid substrates: Hydrogen-termination of solid diamond substratesfollowed procedures outlined by Thoms, et al. (B. D. Thoms, M. S. Owens,J. E. Butler, C. Spiro, Applied Physics Letters. 65, 2957 (1994)).Samples were loaded in a custom-fabricated hydrogen plasma chamberconstructed from a 2-inch quartz tube surrounded by radio-frequencycoils connected to a 13.56 MHz radio-frequency (RF) power source via animpedance-matching circuit. The tube was repeatedly evacuated and purgedwith hydrogen gas, and then heated to approximately 800° C. in 5.0 Torrof flowing H₂. An RF plasma was ignited with a delivered power of 35Watts for 20 minutes. The samples were cooled in the plasma for 15 minand then further cooled in pure hydrogen for 20 min.

Diamond powder: diamond powder was loaded in a metal boat and thenheated at 1023 K in a 1 atmosphere of flowing H₂ for 3 hours in a tubefurnace with a quartz tube liner. The sample was then cooled down toroom temperature in hydrogen. Diffuse reflectance infrared spectroscopymeasurements showed that this treatment removed surface oxides and leftthe diamond powder hydrogen-terminated.

SEM Imaging of Powdered Diamond and TiO₂:

Scanning electron microscopy images of the diamond and TiO₂ powders wereobtained by spin-coating dilute suspensions onto a silicon substrate andthen imaging in a Leo Supra55 VP scanning electron microscope. The SEMimages of the diamond powder showed a substantial dispersion in sizes,with an average of approximately 125 nm, in agreement with themanufacturer's specifications. The TiO₂ nanopowder had particles with anarrower size distribution and an average diameter of approximately 100nm.

Photocatalytic Measurements of Nitrogen Reduction to Ammonia:

Photocatalytic efficiency measurements were performed using ahigh-pressure mercury lamp (Daiel Instrument, model#66921), locatedapproximately 10 inches from the samples. A water absorptive filter wasused to eliminate infrared radiation. Pure N₂ gas was slowly bubbledthrough the quartz reaction vessel (the reduction cell), which containeddeionized water (the fluid of the fluid sample) that was saturated withN₂ via a continuous slow flow. The gas-phase effluent was passed into asecond quartz vessel (the collection cell) containing diluted H₂SO₄ tocapture the NH₃ produced for later analysis. The NH₃ in both thereactant vessel and the capture vessel were measured as described below,almost all the NH₃ was found in the capture vessel.

The production of ammonia was measured using the indophenols blue method(D. F. Boltz, Ed., Colorimetric Determination of Nonmetals, (J. Wileyand Sons, New York, 1978)). A 2 ml aliquot of solution was removed fromthe reaction vessel. To this solution was added 0.100 ml of a 1M NaOHsolution containing 5% salicylic acid and 5% sodium citrate (by weight),followed by addition of 20 μl of 0.05 M NaClO and 20 μl of an aqueoussolution of 1% (by weight) Na[Fe(NO)(CN)₅] (sodium nitroferri cyanide).After 1 hour, the absorption spectrum was measured using a Shimadzu2401PC Ultraviolet-Visible Spectrophotometer. The formation ofindophenols blue was determined using the absorbance at a wavelength of700 nm. Absolute calibration of the method was achieved using ammoniumchloride solutions of known concentration as standards.

X-Ray Photoelectron and Ultraviolet Photoemission SpectroscopyMeasurements:

X-ray photoelectron spectroscopy (XPS) data were obtained using amodified Physical Electronics system equipped with an aluminum K_(α)source, a quartz-crystal X-ray monochromator, and a 16-channel detectorarray. Ultraviolet photoemission spectroscopy (UPS) measurements wereperformed using the same apparatus, using excitation from a He(I)resonance lamp. The electron affinities were calculated from the energywidth (w) of the emission spectrum and the known photon energy (21.2 eV)and diamond bandgap (5.5 eV) using χ=E_(photon)−E_(gap)=21.2−5.5−w.

Results:

FIG. 2 shows the ammonia produced when each of the diamond samples wasplaced into N₂-saturated deionized water and illuminated with light froma high-pressure HgXe arc lamp for the indicated periods of time. For theboron-doped sample results are shown for experiments using asingle-compartment cell and an H-cell design. In the single-compartmentcell the diamond sample has no external electrical connection andtherefore must induce both oxidation and reduction reactions in order tomaintain charge neutrality. In the H-cell geometry the diamond electrodeis contained in one compartment (the reduction cell), and a secondcompartment (the oxidation cell) contains a platinum electrode immersedin a 0.001 M solution of KI in water. The solutions in the twocompartments were connected via a glass frit that provided electricalcontact between the solutions but prevented mixing, and an external wireconnected the diamond and Pt electrodes. In this geometry the diamondneed only induce the N₂ reduction, while charge neutrality is maintainedby oxidation of I⁻ to I₃ ⁻ at the Pt electrode.

In both the stand-alone and H-cell geometries, the boron-doped diamondsample induced a rapid increase in NH₃ production over the first severalhours, followed by a slower rate at longer times. Three types of controlexperiments were also performed: (1) no illumination, (2) illuminationof the sample with Ar replacing the N₂, and 3) illumination of the waterwithout a diamond sample present. These controls all showed nosignificant production of NH₃.

As shown in FIG. 2 a, the rate of NH₃ production for the boron-dopeddiamond was increased several-fold by separating the oxidation andreduction reactions via the H-cell design. This increase occurredbecause the valence band of diamond is relatively shallow (see FIG. 1)making the corresponding valence-band holes relatively poor oxidizingagents. The standard electrochemical reduction potential for thereaction O₂(g)+4H⁺+4 e⁻→2H₂O is E⁰=1.229 V, so that at pH=7 thepotential needed to oxidize water is E=+0.82 V. This is close to, orpossibly even slightly more positive than, the position of the valenceband of diamond. However, when illuminated with photons with hν>5.5 eVthe emission of electrons makes the diamond positively charged,increasing its potential until the conduction band can induce wateroxidation at a rate equivalent to that of the electron emission. Thus,in the single-cell geometry the rate at which nitrogen can be reduced toNH₃ is limited by the need for an oxidation process.

By providing a sacrificial moiety such as I⁻ that can be more easilyoxidized, the reduction reaction can proceed at a faster rate becausethe Pt electrode can rapidly oxidize I⁻ to I₃ ⁻ as necessary to maintaincharge neutrality. While this could also be accomplished in a singlecell, the H-cell avoids interaction of the reaction products andeliminates losses due to absorption of light by the I⁻ solution (KI iscolorless, but it absorbs UV strongly). Similar results were alsoachieved replacing the Pt electrode with Cu²⁺, using the reaction Cu²⁺+2e⁻→Cu_((s)) (E⁰=+0.34 V) as the oxidation reaction; this yielded resultsidentical to those using F.

FIG. 2 d shows that the ability of H-terminated diamond to inducephotocatalytic reduction of N₂ to NH₃ also extends to high-qualityelectronic-grade diamond and, remarkably, even to inexpensive diamondpowder of the type commonly used as a polishing compound (FIG. 2 b),when dispersed in water (˜125 nm average size, 0.1 wt % suspension,stirred). Because the diamond powder and electronic grade diamond arenon-conductive, results for these samples were obtained only in thesingle-compartment cell.

As a point of comparison the photocatalytic activity of ruthenium-loadedTiO₂ catalyst was also measured. This catalyst was prepared using theimpregnation method using a Ru loading of 0.24% by weight, whichprevious studies showed yielded the highest activity for nitrogenreduction (K. T. Ranjit, T. K. Varadarajan, B. Viswanathan, Journal ofphotochemistry and Photobiology A: Chemistry 96, 181 (1996)). FIG. 2 cshows the total yield of NH₃ production after a dispersion (0.1% inwater, stirred) was illuminated under conditions identical to those usedfor the diamond studies. While the Ru/TiO₂ sample showed good activityinitially, it was rapidly deactivated and showed little detectableactivity after ˜15 minutes. Comparison of the data from diamond samplesin FIGS. 2 a-2 b with that from the Ru/TiO₂ sample in FIG. 2 c showedthat the diamond samples were able to produce significantly more NH₃.The loss of activity of Ru/TiO₂ and other metal-activated TiO₂ catalystshas been noted in many previous studies and has been attributed to anumber of factors, including re-oxidation of the ammonia into nitrate ornitrite ions by the photoexcited holes of TiO₂ and the loss of specialsurface sites able to bind N₂ to the catalyst surface. (K. Tennakone, S.Wickramanayake, C. A. N. Fernando, O. A. Ileperuma, S. Punchihewa,Journal of the Chemical Society, Chemical Communications, 1078 (1987);Q.-s. Li, K. Domen, S. Naito, T. Onishi, K. Tamaru, Chemistry letters,321 (1983).) Diamond can provide higher activity both because thebarrier-free photoemission of electrons into the reactant liquidobviates the need for N₂ adsorption to the surface, and also because theshallower valence band of diamond makes it less likely to re-oxidize theproducts of the reduction.

To establish a correlation between the ammonia yield and the excitationwavelength, the same configuration as in FIG. 2 was used, except thatabsorptive filters were included to limit the range of excitationwavelengths. These data, shown in FIG. 3, demonstrated that highactivity was observed only when using short-wavelength light, withwavelengths λ≦230 nm, corresponding to the bandgap of diamond. Longerwavelengths contributed only minimally to N₂ reduction. Given that thephotocatalytic activity of diamond is only induced by veryshort-wavelength photons, it was remarkable that when both wereilluminated with broad-spectrum light diamond still provided a higheryield than Ru/TiO₂, which absorbs a much larger fraction of the incidentlight (λ≦390 nm). The dark control sample showed no detectable ammonia;the value shown represents the detection limit of the indophenol method.

The photoelectrochemical response of the diamond-water interface wascharacterized using transient surface photovoltage (SPV) measurements.In these measurements, the sample was illuminated with pulsed laser (3ns pulse width); emission of electrons or other separation of chargeinduced a transient photocurrent that was measured using a second,capacitively coupled electrode placed ˜25 μm away; integration of thecurrent yielded the amount of charge transfer, Q_(trans). FIG. 3 ashowed the time evolution of Q_(trans) (inset) and the magnitude of themaximum (Q_(max)) as a function of wavelength for a boron-doped diamondin contact with N₂-satured water. The wavelength-dependent measurementswere normalized by photon energy. The sign of the charge transfer inFIG. 3A corresponds to an accumulation of negative charges at thediamond-water interface. The wavelength dependence shows that atwavelengths λ>270 nm there was very low response, while as thewavelength was reduced below 270 nm the emissive properties increasedrapidly and then saturated at λ<230 nm. This profile is very similar tothat reported previously for diamond photoemission in to vacuum (J. B.Cui, J. Ristein, L. Ley, Physical Review B 60, 16135 (December, 1999)).A small amount of signal was observed at wavelengths longer than ˜300nm; this likely arose from optically induced transitions involvingdefects and/or impurities within the sample, which is dark in color dueto the presence of a small amount of graphitic impurities.

The significance of diamond's negative electron affinity was illustratedby comparing the reactivity of diamond with different surfaceterminations. While H-terminated surfaces have NEA, oxidized diamondsurfaces have positive electron affinity (PEA) because the dipolesassociated with the C—O surface bonds induce a barrier to electronemission; consequently, the photoemissive properties of diamond aregreatly attenuated when oxidized. To demonstrate the influence ofsurface termination on the photocatalytic reduction of N₂, H-terminatedsamples were compared with samples that had been intentionally oxidizedby exposure to ozone produced by short-wavelength (185 nm) ultravioletlight in air, leaving them O-terminated. FIG. 4A shows the relativeyield of ammonia produced by H-terminated and O-terminated samples ofhigh-quality electronic-grade diamond, boron-doped electrochemical gradediamond, and diamond powder, measured after 2 hours illumination inwater. In all three cases the H-terminated surfaces had highphotocatalytic activity, while the oxidized surfaces showed much lowerreactivity. These results show excitation of electrons to the conductionband alone was not sufficient to get good activity—it is also necessaryfor the electrons to have an efficient pathway for emission from thediamond to the adjacent reactant liquid, which is greatly facilitated bythe negative electron affinity of the H-terminated samples.

While H-terminated diamond retains its photocatalytic activity forseveral days of constant illumination, the activity was graduallyreduced. FIG. 4 b shows X-ray photoelectron spectroscopy data fromboron-doped diamond samples after various illumination times in water;these show that the initially H-terminated sample can undergo a gradualincrease in surface oxygen and also the appearance of a small amount ofsurface nitrogen (400.6 eV, typical of amino groups) only in verylong-time illuminated samples.

The electron affinity from equivalent samples was measured usingvalence-band photoemission spectroscopy. As shown in FIG. 4 c, theH-terminated sample had negative electron affinity, χ˜−0.8 V, while thefully oxidized sample has a positive electron affinity of ˜1.5 V. Thecrossover from negative to positive electron affinity occurred afterapproximately 2 hours of illumination. This suggests that the observeddecrease in catalytic efficiency after illumination for more thanseveral hours is a direct consequence of a gradual loss of NEA due tooxidation of the surface.

Example 2

This example demonstrates the use of hydrogen-terminated diamond as aphotoreduction catalyst for the reduction of CO₂ to CO.

Materials and Methods:

Samples:

Natural diamond powder was purchased from Microdiamant AG, LengwilSwitzerland. The diamond powder had a nominal average particle size of150 to 250 nm. The diamond powder was suspended in 0.1 M KCl and 0.1 mMNa₂SO₃ solution with nanopure water. In each case the diamond sampleswere hydrogen-terminated before use as described below.

Hydrogen-Termination:

The diamond powder was hydrogen terminated by heating at 750° C. under ahydrogen atmosphere for two hours. Approximately 20 mg of H-terminateddiamond powder was sonicated in 20 mL of this solution for 15 minutes.

Photocatalytic Measurements of Carbon Dioxide Reduction to CarbonMonoxide:

The diamond suspension (approximately 15 mL) was then placed in a quartzvial, sealed with septum, and stirred while CO₂ (99.999% purity) wasbubbled through for 30 minutes. The septum was then removed and theglass vial was placed on an infrared gas cell. The gas cell had calciumfluoride windows on it, sealed with black wax. An FTIR spectrum of thecell was taken. Then, the quartz vial with the diamond suspension wasirradiated with deep ultraviolet radiation from a xenon lamp through awater filter. The irradiation took place after 18 hours. The diamondsuspension was stirred continuously.

Results:

FTIR spectra of the sample before and after irradiation showed thatinitially only CO₂ was present in the solution, but after irradiation COwas also present. Control experiments were conducted to confirm theresult.

In the first control experiment, a sample was run with the same amountof the hydrogen-terminated diamond powder in KCl/Na₂SO₃ solution, butwithout exposure to UV radiation. Once the diamond was suspended in thesalt solution the quartz vial was wrapped in foil to prevent any lightfrom entering. After bubbling with CO₂ for 30 minutes, the vial wasattached to the infrared gas cell and the suspension was stirred for 43hours. FTIR spectra were taken at 0 hours and at 43 hours. No CO peakwas observed in the spectra before or after the period of stirring.

In the second control experiment, a sample of the salt solution withoutthe diamond was run. In this experiment CO₂ was bubbled through the saltsolution in a quartz vial for 30 minutes. The vial was attached to theFTIR gas cell and the CO₂ solution was irradiated with deep UV radiationfor 18 hours. No CO peak was present in the spectra before or after theirradiation.

Example 3

This example demonstrates the use of nitrogen-doped, hydrogen-terminateddiamond as a photoreduction catalyst for the reduction of N₂ to NH₃.

Materials and Methods:

Samples:

Samples of nitrogen-doped diamond were obtained from Applied Diamond,Inc. (Wilmington, Del.).

Ammonia Production Using Nitrogen-Doped Diamond:

After hydrogen-termination, the diamond (0.5 cm×0.5 cm) was placed in aquartz tube and flushed with 50 standard cubic centimeters per minute(sccm) ultrahigh purity nitrogen. The area of the nitrogen-doped diamondwas smaller than that of the diamond samples used in Examples 1 and 2(e.g., 0.25 cm² area compared with 1 cm² for the other samples).

Results:

The amount of ammonia produced per unit area of illuminated diamond wasapproximately 5-6 times higher than that produced by the boron-dopeddiamond or electronic-grade diamond, indicating that nitrogen-dopeddiamond had higher photocatalytic efficiency than the other forms ofdiamond.

The data for the measured ammonia yield over a period of four hours ofillumination are shown in Table 1.

TABLE 1 Time (h) Ammonia Yield (μg) 0 0 1 0.22 3 1.18 4 1.34

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”. Still further, the use of “and” or “or” is intended to include“and/or” unless specifically indicated otherwise.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

1.-20. (canceled)
 21. A method for the photoreduction of molecules, themethod comprising: (a) illuminating a fluid sample comprising themolecules to be reduced and a hydrogen surface-terminated diamondelectrode having a negative electron affinity with light comprising awavelength that excites electrons from the valence band of the diamondto the conduction band of the diamond, thereby inducing the emission ofthe excited electrons from the diamond into the fluid sample, whereinthe emitted excited electrons induce the reduction of the molecules toform a reduction product, further wherein substantially all of thereduction product is formed via the emitted excited electrons; and (b)collecting the reduction product, wherein a counter electrode isimmersed in the fluid sample.
 22. The method of claim 21, wherein avoltage source is in electrical communication with the hydrogensurface-terminated diamond electrode and the counter electrode.
 23. Themethod of claim 22, further comprising applying a voltage between thehydrogen surface-terminated diamond electrode and the counter electrode.24. The method of claim 23, wherein the applied voltage is sufficient tomove the emitted excited electrons away from the hydrogensurface-terminated diamond electrode.
 25. The method of claim 23,wherein the voltage is less than or equal to 2 V.